(a) Field Of The Invention
This invention relates to apparatus for determining and controlling the mass rate of flow of air flowing in an air intake conduit of an internal combustion engine and more particularly to such apparatus which is adapted to maintain a difference between the air pressures existing on opposite sides of an air flow detection valve located in the air intake conduit at a control value which is adapted to be varied in inverse proportion to the specific gravity of the air being measured, thereby ensuring the detection of the mass rate of flow of the intake air in terms of the area of opening of the air flow detection valve.
(b) Description Of The Prior Art
In internal combustion engines, e.g. for automobiles, it is particularly important to engine efficiency and exhaust gas pollution control to accurately control the mass ratio of air to fuel so as to keep it at a constant value. To this end, a high precision measuring device for measuring the amount of intake air is required. Generally, the conventional intake air flow determining and control apparatus for this purpose is known as the so-called area flowmeter which comprises a throttle valve placed in an air intake conduit and a flow detection valve placed upstream of the air throttle valve, the two valves defining an intermediate chamber, and means for controlling the pressure in said intermediate chamber so as to keep substantially constant the difference in the pressures existing on opposite sides of the flow detection valve, so that the amount of intake air is proportional to the area of opening of the flow detection valve, the volume rate of flow of air being thus detected by that area of opening. The control of the flow detection valve by this system is performed by employing a pressure difference control servo-mechanism based on the feedback system wherein when the difference in the pressures existing on opposite sides of the valve deviates from a predetermined control value, the deviation is detected by a pressure-responsive diaphragm of the servo-mechanism, the detected value being then amplified by a fluid mechanism, the resulting amplified output being used to increase or decrease the degree of opening of the flow detection valve so as to bring the pressure difference to the predetermined control value.
However, the intake air varies in its specific gravity with its pressure and temperature. Therefore, in order to accurately measure the mass rate of flow of air, a mechanism which compensates for temperature and pressure is required. Such mechanisms which have heretofore been used are in two types; electrical and mechanical. In either type, there are problems in terms of accuracy and cost. There has already been proposed a compensation mechanism of this kind using the above-described feedback control mechanism having incorporated therein a compensation section so designed that compensation for pressure is made by a pressure sensitive device and compensation for temperature is made by a temperature sensitive device. The mechanism, however, has problems in terms of the number of parts and construction.